Increases in the use of marijuana have led to the development of assays for the detection of the primary active constituent of the plant, .DELTA..sup.9 -tetrahydrocannabinol (THC), and, more particularly, metabolites of THC in urine and blood samples. These assays employ the use of labeled cannabinol derivatives in conjunction with antibodies against metabolites of the drug.
In practice, a blood or urine sample suspected to contain cannabinol metabolites (including glucuronides and other conjugation products) is contacted with the antibodies in the presence of a labeled derivative. To the extent that cannabinol metabolites are present in the sample, there will be competition for binding to the combining sites of the antibodies, and the amount of the labeled derivative bound will be reduced in proportion to the degree of such competition.
Descriptions of some representative immunoassays can be found in O'Connor et al., J. Anal. Toxicol. 5:168 (1981), Law et al., J. Anal. Toxicol. 8:14 (1984) and Childs et al., J. Anal. Toxicol. 8:220 (1984). In all of these references, it is the displacement of some of the labeled cannabinol derivative by metabolites in the assay samples that is the basis of the assays described. Superior assay results will be obtained when the labeled derivative is specifically recognized by the antibodies and yet easily displaced by the various products of cannabinol metabolism.